I'm pretty sure that calves recover more quickly than most muscle groups and can be trained twice each week. Your body also reacts better to variety than the same exercise done the same way each time. Try to vary your number of reps, sets, and amount of weight. Sometimes you can go lighter and do 10 reps per set. Sometimes go heavier and only 3 reps per set. I wouldn't recommend 12-15 reps per set for building muscle mass. Also, do your calf exercises at the very beginning of your workout. You'll have the most energy and will be able to tear down the muscles more at the beginning than later in the workout. You can also vary which part of your calves are worked by sometimes pointing your toes in or out to focus on the inside and outside heads of the calves.
I used to work out my forearms by grabbing some big dumbbells (around 100 lbs.) and walking around until just before my grip strength gave out (i.e. I didn't just drop them at the end). I did wrist curls in the past, but I don't care much for them.